Actions: Pin version of upload-artifact action (#549)

Author: adityasharad

.github/workflows/main.yml

@@ -37,7 +37,7 @@ jobs:
           cp dist/*.vsix artifacts
 
       - name: Upload artifacts
-        uses: actions/upload-artifact@master
+        uses: actions/upload-artifact@v2
         if: matrix.os == 'ubuntu-latest'
         with:
           name: vscode-codeql-extension

.github/workflows/release.yml

@@ -63,7 +63,7 @@ jobs:
       # This is just in case the release itself fails and we want to access the built artifacts from Actions.
       # TODO Remove if not useful.
       - name: Upload artifacts
-        uses: actions/upload-artifact@master
+        uses: actions/upload-artifact@v2
         with:
           name: vscode-codeql-extension
           path: artifacts